The present invention relates to electrometallurgy, and, more particularly, to plasma-arc-furnaces for remelting metals and alloys; it may be advantageously employed for manufacturing ingots of pure metals, steels and alloys.
The rapidly advancing modern industries, such as aerospace engineering, atomic power engineering, chemistry, electronics and cryogenics, impose ever more stringent requirements on the purity of metals and alloys. At present, metals and alloys are customarily remelted in electroslag, vacuum-arc, electron-beam and plasma-arc furnaces with a view to improving their purity and processing behaviour.
These methods are all characterized by that the operations of melting, teeming and crystallization are synchronized, and the ingot is formed in a water-cooled copper crystallizer, with the molten metal and heat being continuously supplied through the open surface of the molten pool. The molten metal is transferred from the end face of the billet being remelted or the electrode to the crystallizer in small portions (drops) uniformly distributed in time, thereby providing for an extremely extended reactive surface area.
The plasma-arc technique of remelting offers the largest scope of means of affecting molten metal, such as gas, slag, vacuum and directed crystallization, for producing homogeneous ingots with a high surface quality negligibly low levels of nonmetallic inclusions and gases.
As far as the manufacture of large ingots is concerned, one of the most promising techniques of plasma-arc remelting is one wherein the plasma generator is formed is a hollow consumable electrode wherein the cavity defines a channel for the passage of plasma-forming gas to the plasma-burning zone. This technique of plasma-arc remelting features low rates of power and plasma-forming gas consumption, as well as high productivity and excellent quality of the product metal.
The plasma-arc furnace for remelting metals and alloys operating on the foregoing principle comprises a crystallizer and a melting chamber which are put together, defining a closed air-tight space. The hollow consumable electrode is attached to a hollow rod entering the chamber in such a manner that its axis coincides with the axis of the crystallizer. The furnace is provided with a plasma-forming gas supply and flow rate control system, and the hollow rod and the crystallizer are coupled to the opposite poles of the power source.
The above-described furnace, however, features an important disadvantage detracting from the efficiency of the process and adversely affecting its economics: its design makes it difficult to eliminate the shrinkage cavities in the ingot at the end of the melting process.
The available methods for eliminating shrinkage cavities, which are employed in the electroslag, vacuum-arc, electron-beam and ordinary plasma-arc remelting processes whereby the electric power is gradually decreased at the end of remelting, proved of little use. The reason for this should be sought in the fact that, as the power supplied to the plasma discharge generated by the hollow consumable electrode is reduced, the burning stability deteriorates sharply so that the arc is destabilized. As a result of substantial fluctuations of current intensity and voltage across the arc, the process of uniform crystallization of the ingot head is disrupted, giving rise to shrinkage cavities at a depth equal to 0.3 to 0.5 of the ingot diameter. Thus, this portion of the ingot has to be cut off, as a rule, reducing the yield of metal and detracting from the economic and technical efficiency of the process.